Ethene (synonym: ethylene) is the simplest olefin (alkene). Its empirical formula is C2H4; it therefore has two carbon atoms and is also referred to as C2 olefin. Because of its high reactivity, ethene is an important synthesis unit in the chemical industry.
By far the greatest volume of ethene is polymerized to polyethylene, one of the most widely used and ubiquitous bulk polymers, usually in the form of packaging film. Polyethylene is a chain polymer in which the ethene monomer[—CH2—CH2—] is repeated many times as a chain member. A polyethylene chain polymer therefore has a very large number of carbon atoms, far more than 96.
It is also possible to prepare other olefins having four, six or eight carbon atoms from ethene. This is accomplished by way of oligomerization. In ethene oligomerization, ethene reacts essentially with itself and forms olefins having more than two carbon atoms, called oligomers. For example, two ethene molecules can react to give a butene molecule, i.e. an olefin having four carbon atoms (C4 olefin for short). This reaction is also referred to as dimerization; butene is the dimer of ethene. Three ethene molecules together form a hexene (C6 olefin) (trimerization), and four ethene molecules form a C8 olefin as oligomer (tetramerization). In parallel, two butenes formed beforehand can form an octene. This is because several reactions proceed simultaneously in parallel in the oligomerization: the primary reaction is that of ethene with itself. Secondary reactions proceed between ethene and already formed ethene oligomers or oligomers with one another.
Compared to polymerization, oligomerization gives rise to molecules having a much smaller number of carbon atoms. The limit can be set at sixteen carbon atoms. Another important feature of an oligomerization is that new olefins are formed in turn from olefins and not saturated chains.
All in all, an ethene oligomerization in the context of this invention is understood to mean the chemical reaction of ethene to form olefins having a carbon number of four to sixteen.
Ethene oligomerization is practiced in industry for preparation of C4, C6 and C8 olefins. These serve in turn as reactants for more complex chemicals, for example higher alcohols, carboxylic acids and esters.
From a chemical engineering point of view, oligomerization processes can be divided into those which are conducted in the gas phase and those which proceed in liquid phase. In addition, heterogeneously catalyzed operations are distinguished from homogeneously catalyzed processes.
In gas phase operations, the oligomerization is conducted under conditions under which ethene is gaseous. The oligomers may then likewise be gaseous or else liquid.
In the liquid phase oligomerization, the ethene is introduced into the reactor in liquid form. Since ethene is liquid only under very high pressure, the liquid phase oligomerization of ethene is usually implemented by dissolving gaseous ethene in a liquid solvent and effecting the oligomerization in the liquid solvent. The oligomers are then also present in the solvent. The advantage of liquid phase oligomerization over gas phase oligomerization is the better exploitation of the reactor volume and better removal of the heat of reaction with the solvent (oligomerization is highly exothermic!). Overall, liquid phase oligomerization achieves a better process intensity than gas phase oligomerization.
Disadvantages of liquid phase oligomerization are the need for a solvent and the difficulty of product removal: product removal is the recovery of the oligomers from the reaction mixture which comprises, as well as the desired oligomers, also unconverted ethene, any solvent, and catalyst. If the process is homogeneously catalyzed, the catalyst is dissolved in the same phase as the reactant and the oligomerizate. If the catalyst cannot remain in the product mixture, it has to be removed accordingly. This presents additional complexity from a chemical engineering point of view.
By contrast, catalyst removal is simpler in heterogeneously catalyzed operations in which the catalyst is present in a different phase from the reactants, generally in the solid state. Both in the heterogeneously catalyzed liquid phase oligomerization and in the heterogeneously catalyzed gas phase oligomerization, the solid catalyst remains in the reactor, while the fluid product mixture is being drawn off from the reactor.
The fact that the heterogeneous catalyst remains in the reactor enables constant reuse thereof. However, this has the consequence that the catalyst loses activity with increasing service life and increasingly forms unwanted by-products. This is probably because the active sites of the catalyst become covered with deposits, such that there is no longer any contact with the ethene. These deposits are probably longer-chain by-products extending as far as low molecular weight polyethylene and/or catalyst poisons consisting of N-, O- or S-containing molecules. It is also possible for active metals in the catalyst to be oxidized in the oligomerization and hence to lose activity.
Fortunately, the deactivation of heterogeneous oligomerization catalysts is reversible: for instance, it is possible to reactivate the catalyst from time to time, as a result of which it very substantially regains its initial performance.
The prior art describes some methods of reactivating oligomerization catalysts: for instance, DE102009027408A1 describes the regeneration of a heterogeneous catalyst based on nickel oxide, silicon dioxide and aluminum dioxide. It is used preferentially in the oligomerization of C3 to C6 olefins. For removal of organic deposits, they are burnt off with a hot oxygenous gas stream. This is done in an oven. A disadvantage of this method is that the catalyst has to be deinstalled from the reactor and transferred into the oven for reactivation. On completion of regeneration, the catalyst has to be reinstalled in the reactor. This is associated with a comparatively large amount of manual work and causes long shutdown times of the oligomerization plant.
A simpler method in that respect is the regeneration, described in EP0136026B1, of a heterogeneous zeolite catalyst used in the mixed oligomerization of C2 and C3 olefins. The regeneration here is effected in situ, i.e. at the normal location of the catalyst, namely in the reactor. For the regeneration itself, the olefin stream to the catalyst is shut down and the reactor with the catalyst present therein is purged with a hot oxidizing gas. Regeneration in situ has the crucial advantage that the catalyst need not be deinstalled from the reactor and then reinstalled for regeneration. A disadvantage of this process is that the reactor has to be designed so as to have sufficient thermal stability to withstand the hot regeneration gases as well. Provided that the reactor is designed for gas phase oligomerization in any case—as is the case in EP0136026B1—the extra costs are acceptable. By contrast, making a reactor optimized for liquid phase oligomerization capable of withstanding regeneration with hot gases is associated with considerable extra costs. Furthermore, it is questionable whether all heterogeneous catalysts can be regenerated with hot gas, since the examples of EP0136026B1 are concerned solely with zeolitic catalysts. Particularly those catalysts containing nickel, chromium, iron or titanium as active metal behave completely differently in the oligomerization from the zeolites consisting solely of aluminum oxides and silicon oxides. It is therefore to be expected that such catalysts will need a different regeneration.
The regeneration of a chromium-based catalyst which is used in the liquid phase oligomerization of ethene is described in WO2014082689A1. However, this concerns a homogeneous catalyst system which is regenerating ex situ, i.e. outside the reactor.
There are also examples in the prior art of in situ regeneration of catalysts which are used in the liquid phase oligomerization of ethene (WO2011112184A1, WO2010110801A1), but these are also all homogeneous systems.
The in situ regeneration of heterogeneous catalysts which are used in the liquid phase oligomerization of ethene is not currently known to have been described to date.